As much as I hate to use the old cliche, this is pretty much a case of, "If you have to ask how, you probably shouldn't be doing it." Mirror adjustments are delicate and must be done precisely. It doesn't take much misalignment at all of the mirror (and the attached secondary mirror) to render a camera's AF system completely useless. The optical path from the lens to the AF sensor via the secondary mirror must be exactly the same length as the optical path from the lens to the camera's sensor. The same is true of manual focus: The optical distance from lens via the mirror to the view/focusing screen at the top of the light box just below the prism must match the optical distance from the lens to sensor.

Your real question is whether the camera is worth enough to spend on the repair or if you would be better served to replace the camera.